The present invention relates to an improvement to a robot joint angle detecting system including an absolute type rotational angle detector and an incremental type rotation number detector coupled via a reduction gear to an axis of rotation of a joint of a robot.
For control of a robot, accurate detection of the rotational angle of the axis of each joint is vital. In the above-mentioned detecting system, the detectors detects the rotational angle and the rotation number which vary at a higher rate than the axis of the rotation of the joint because of the presence of the reduction gear. If the reduction ratio is represented by 1/R, the joint angle (joint rotational angle) .theta. is given by: EQU .theta.=(360.degree..times.N+.theta.')/R (1)
where N represents the number of rotations of the detector, and .theta. represents the rotation angle of the rotational angle detector.
For instance, if R=10 (for the sake of simplicity), N=7, .theta.'=27.degree., then EQU .theta.=(360.degree..times.7+27)/10=255.7.degree.
An example of the increment type rotation number detector comprises a pulse generator producing pulses once per rotation of the axis on which the pulse generator is mounted and a counter counting the pulses. The absolute type rotation angle detector may comprise a potentiometer or a resolver with an encoder for producing a digital signal indicative of the detected rotational angle.
When the detecting system is connected to a power supply or when the power supply is restored after interruption, the detectors need to be calibrated. More specifically, the joint is rotated to the original or a reference position, and the rotational angle of the rotational angle detector is made to be zero and the counter of the rotation number detector has to be cleared to zero.
Such a calibration has to be effected each time the power supply is restored after interruption of the power supply which may occur if the power fails or if the system is disconnected from the power supply. This lowers the efficiency of work. Moreover, it is sometimes difficult to return the joint to the reference position.
Furthermore, during assembly of the rotation number detector and the rotational angle detector, the zero position alignment must be made accurately, since otherwise error occurs in the detection of the joint angle in the vicinity of 0.degree. of the rotational angle detector.